Dry cleaning is a major industry throughout the world. In the United States alone, there are more than forty thousand dry cleaning machines. In Europe, there are more than 60,000 dry cleaners. More than 85% of these dry cleaners use machines constructed for use with a perchloroethylene solvent (“PERC”). While PERC remains a good cleaning solvent, it presents several major health and environmental hazards, evidenced by numerous lawsuits for ground contamination and legislation for controlling and/or eliminating the use of PERC as a dry cleaning solvent.
Despite its health and environmental hazards, PERC remains the most widely used dry cleaning solvent worldwide. Because the majority of dry cleaners use PERC as a cleaning solvent, the majority of dry cleaning machines are designed specifically for use with PERC, which has certain characteristics that influence the design of the equipment and the method for regenerating the solvent. For example, PERC has a boiling point of 256° F., thereby enabling use of an atmospheric still for solvent regeneration. Also, PERC has high solvency. Solvency is typically reported as a Kauri Butanol Value (“KBV”), and PERC has a KBV of over 90. The KBV is a measure of solvency and the ability of a solvent to solubilize hydrophobic impurities. PERC's high solvency enables the solubilization of many impurities. Consequently, distillation is an excellent method of PERC regeneration because the solubilized impurities are typically not volatile and therefore become part of the waste-stream or non-volatile residue (“NVR”). The NVR is treated as hazardous waste, and its disposal is regulated.
In other parts of the world, such as Japan, which has over 60,000 dry cleaners, petroleum distillates are widely used as the cleaning solvent. These petroleum distillates have high boiling points ranging from 300° F. to 400° F., making vacuum distillation necessary to reduce the boiling temperature. Systems using vacuum distillation are typically the most expensive dry cleaning systems. Also, petroleum distillates have low flash points, and are therefore strictly regulated to prevent fire and explosion.
Petroleum distillates have solvencies ranging from 27 to 40 KBV. While these petroleum distillates have solvencies much lower than that of PERC, they have proven to sufficiently solubilize many of the hydrophobic impurities that are present in the dry cleaning process. However, regeneration of petroleum distillates by distillation also creates a hazardous waste stream subject to regulated disposal. Also, petroleum distillates are categorized as volatile organic compounds (“VOCs”) and present both health and environmental concerns. Like with PERC, distillation is an excellent method for regenerating petroleum distillates because the solubilized impurities are typically not volatile and therefore become part of the waste-stream or non-volatile residue (“NVR”). The NVR is treated as hazardous waste, and its disposal is regulated.
In addition to distillation, filtration of these solutions also produces hazardous waste subject to regulated disposal. Prior to 1970, powder filters with diatomaceous earth were used for filtration. During the 1970s, however, these powder filters were widely replaced with cartridge filters. Then, in the 1980s, the U.S. Environmental Protection Agency (“EPA”) categorized used cartridge filters as hazardous waste, making dry cleaners liable for the required special treatment and handling.
Regeneration of cleaning solvents through filtration and distillation is the largest source of hazardous waste in modern dry cleaning plants. This hazardous waste is both expensive to dispose of and is extremely unhealthy for the environment. As a result, the dry cleaning industry has focused its efforts on reducing this hazardous waste while maintaining good cleaning quality.
Due to environmental and government regulatory restraints, the industry's efforts have concentrated on developing alternatives to PERC and petroleum distillates. The search for alternative solvents has focused on environmental friendliness, functionality and economic practicality. These efforts led to the introduction of high flash point hydrocarbons, liquid carbon dioxide, glycol ethers, and more recently, siloxanes. Because siloxanes have only recently been introduced, systems and methods designed for their use as dry cleaning solvents are still needed.